1. Field of the Invention
The present invention relates to a differential amplifier and a radio system with a differential amplifier.
2. Description of the Background Art
High-frequency amplifiers are required for a multitude of applications. Thus, high-frequency amplifiers are used to amplify radio signals that are received or to be transmitted. Furthermore, high-frequency amplifiers are used in sensor systems, mobile telephones, or radar installations. It is moreover sufficient for several applications, that the amplifier amplifies only a narrow frequency band within the range of an operating frequency. This type of amplifier can also be called a selective amplifier.
A differential circuit is disclosed, for example, in “Analoge Schaltungen” (Analog Circuits), Seifart, 4th edition, Verlag Technik Berlin, 1994, pages 107ff. The output voltage of the differential amplifier is proportional to the differential voltage between two input terminals. Common-mode voltages, which act at both inputs with the same amplitude and phase position, are not amplified by the ideal differential amplifier. The differential amplifier receives advantageous properties from its largely symmetric structure. The emitters of the two input transistors are thereby typically connected to one another and to a constant-current source.
A low-power 2.4-GHz transmitter/receiver—CMOS IC with a differential amplifier is known from IEEE Journal of Solid-State Circuits, Vol. 38, No. 2, February 2003, pages 176ff. A 5.2-GHz low-noise amplifier in 0.35 μm CMOS technology with a differential amplifier is known from IEEE Journal of Solid-State Circuits, Vol. 38, No. 4, April 2003.